During development, sensory neurons form highly specific connections between structures in the periphery and neurons in the central nervous system. The hypothesis of "peripheral specification" suggests that developing sensory neurons acquire chemical labels from their peripheral targets that enable their central axons to establish the appropriate synaptic connections. This hypothesis will be tested by forcing sensory neurons to innervate novel peripheral targets and examining their connections with spinal neurons. Three separate series of experiments will be undertaken: (1) Transposition of muscle nerves in juvenile bullfrogs. Muscle afferents will be forced to innervate a foreign muscle, by transposing the muscle nerve, and to reestablish synaptic connections in the spinal cord, by transecting the dorsal root. The central connections will be examined to determine if they are appropriate to the new peripheral target. (2) Transplantation of dorsal root ganglia in bullfrog tadpoles. The possibility that cutaneous afferents can be transformed into muscle afferents will be examined by transplanting dorsal root ganglia that normally only innervate skin to a position where they can innervate both muscles and skin. (3) The influence of peripheral tissues on sensory neurons grown in vitro. Dorsal root ganglia will be co-cultured with explants of spinal cord in the presence of different peripheral tissues to see if the tissues produce factors that influence the connections of sensory neurons onto spinal neurons. By showing how developing neurons are influenced by interactions with their targets, these studies will provide insight into mechanisms that are likely to be of general importance in nervous system development.